Oils are used as lubrication fluids in engines for land vehicles, marine and air transportation, electric power generation and other internal combustion engine applications. Oils are also used to lubricate sliding contact surfaces in machines, and they are used as heat transfer fluids in various equipment applications. In many of these engine and machine applications, the oil is heated well above ambient temperatures in an oxidizing environment and subjected to high shear stresses which chemically degrade the oil. The oil may also be subjected to contamination from combustion or other external sources. Thus, the initial composition of the oil, that provided its specified lubricating properties, is gradually altered and its lubricating characteristics degraded.
One group of lubricants includes petroleum oils which are widely used in large volumes as lubricants in automotive engines and a wide range of other engines. Such petroleum oils, produced as refined distillation products of crude petroleum, generally range from low viscosity, with molecular weights as low as 250, to very viscous lubricants, with molecular weights up to about 1000. The physical properties and performance characteristics of such engine lubricants depend on the relative distribution of parafinic, aromatic, and alicyclic (naphthenic) components. Depending upon the engine application, these refined oils are formulated to contain special additives such as oxidation inhibitors, rust inhibitors, anti-wear and extreme pressure agents, friction modifiers, detergents, pour-point depressants, viscosity-index improvers, foam inhibitors, and dispersants for contaminants. Commercial engine oils are formulated to contain representatives of most or all of these classes of additives. Apart from petroleum-based or mineral-based lubricants, synthetic molecular carbon-containing and/or silicon-containing lubricant oils have been developed for automotive applications and other machine or mechanism lubricant requiring applications.
The development of lubrication oils is a sophisticated technology, but techniques for determining when lubricating oil, in use in an engine or other mechanism, has reached the end of its useful life are not so advanced. Typically, a fixed operating mileage, time, or an algorithm based on engine operating conditions is used as an indication that the oil is approaching the end of its useful life and should be changed. These indicators are usually specified to underestimate the remaining life of the oil so as to avoid damage to the engine or other mechanism. There is a need for a more accurate method for determining the remaining useful life of lubrication oil in a mechanism. Such a method would enable the useful life of the lubricant to be utilized without discarding the material too soon, and would avoid using the fluid too long to the detriment of the host machine or engine.